Blood moves around in the body in the veins and arteries. Blood carries oxygen from place to place and also carries carbon dioxide from place to place. Perfusion is the process of blood being delivered to a capillary bed in a biological tissue. Blood includes water. Since water is made from hydrogen and oxygen, water is particularly susceptible to being excited by magnetic resonance imaging (MRI) apparatus where radio frequency (RF) energy applied at a specific frequency and in specific orders in a controlled magnetic field produce nuclear magnetic resonance (NMR) in resonant species (e.g., hydrogen). The NMR may occur in the hydrogen atoms, which may be referred to as “spins.” These attributes of blood led to the development of arterial spin labelling (ASL).
In ASL, water in arterial blood is magnetically labeled and then the effect produced by transiting or transited arterial blood is imaged. Since the blood moves around the body, the blood may be magnetically labeled in a first place and then produce an effect that is imaged in a different place. For example, arterial blood water may be magnetically labeled outside a region of interest (RoI). The blood may then move into the RoI where it may, for example, exchange with tissue water. The inflowing labeled spins may change the total tissue magnetization in the RoI or in a portion of the RoI. This change in magnetization may be detectable by MRI. The time between when the spins are labelled and the time when an image is taken is called the transit time because it is the time during which the labeled spins transit from place to place. The time during which the spins are labeled is called the tag time because it is the time during which the spins are “tagged” for use in imaging.
Conventional MRI produces images that may vary between scanners, technologists, or scan settings. Conventional MRI images are interpreted qualitatively, which produces subjective variability in diagnosis. Unlike conventional MRI, magnetic resonance fingerprinting (MRF) provides consistent quantitative parameters maps, which eliminates the variability found in conventional qualitative images. Quantitative parameter maps also reduce or eliminate subjectivity in diagnosis. In MRF, unique signal time courses are generated for pixels. The time courses evolve based on the properties of the material subjected to MRF including T1 and T2, T1 being spin-lattice relaxation, and T2 being spin-spin relaxation.
The signal time course can be matched to an entry in a dictionary. The dictionary may be, for example, a collection of time courses calculated using a range of possible property values in light of quantum physics properties that govern the signal evolution. Performing MRF for multiple pixels yields maps of material properties of interest. MRF may be more efficient than other proposed quantitative methods. In addition, MRF quantifies multiple parameters in a single MR acquisition.
MRF assumes that different materials and different spatial locations have different signal evolutions. In MRF, different materials may be separated by varying user-controllable MR settings including flip angle (FA) and acquisition periods in, for example, a pseudo-random fashion. Randomized encoding may be used to separate different spatial locations.
MRF employs a series of varied sequence blocks that simultaneously produce different signal evolutions in different resonant species (e.g., tissues) to which radio frequency (RF) energy is applied. MRF sequence blocks may vary widely, either non-linearly, randomly, and/or pseudo-randomly. Since the sequence blocks may vary widely, the resulting signal evolutions may also vary widely.
The term “resonant species”, as used herein, refers to an item (e.g., water, fat, tissue, material, blood, arterial blood water, hydrogen in arterial blood water) that can be made to resonate using NMR. By way of illustration, when RF energy is applied to a volume that has bone and muscle tissue, then both the bone and muscle tissue will produce an NMR signal. However the “bone signal” and the “muscle signal” will be different and can be distinguished using MRF. The different signals can be collected over a period of time to identify a signal evolution for the volume. Resonant species in the volume can then be characterized by comparing the signal evolution to known evolutions. Characterizing the resonant species may include identifying a material or tissue type, or may include identifying MR parameters associated with the resonant species. The “known” evolutions may be, for example, simulated evolutions or previously acquired evolutions. A large set of known evolutions may be stored in a dictionary.
Characterizing the resonant species can include identifying different properties of a resonant species (e.g., T1, T2, diffusion resonant frequency, diffusion co-efficient, spin density, proton density). Additionally, other properties including, but not limited to, tissue types, materials, and super-position of attributes can be identified. These properties may be identified simultaneously using MRF, which is described in U.S. Pat. No. 8,723,518 “Nuclear Magnetic Resonance (NMR) Fingerprinting” and in Magnetic Resonance Fingerprinting, Ma et al., Nature 495, 187-192 (14 Mar. 2013), the contents of both of which are incorporated herein by reference.